One of the problems of modern semiconductor technology is the need for rapid determination of the thickness of transparent thin films on semiconductor substrates. Such measurements, to a high level of accuracy, are becoming increasingly significant as thinner films are being deposited, grown or sputtered on semiconductor substrates. It is especially important to conduct such a test in a non-destructive manner.
Therefore, it is an objective of the present invention to provide a system for non-destructive film thickness measurement.
Another objective of this invention is to provide a high accuracy film thickness measurement system, capable of thickness measurements accurate to the order of several Angstroms.
Yet another objective is to provide a film thickness measurement system capable of operating at high speeds, so that the processing sequence of wafers is not retarded by the testing sequence.
Another objective herein is to provide real time film thickness measurements, so that in the event of detection of some slight or minimal abnormality, a wafer can be further tested over an expanded portion of its surface to determine the extent of the process abnormality.
The basic theory on which film thickness measurement proceeds utilizing analysis of visible reflectance spectra, and the data reduction algorithm used in this measurement sequence are expressed in a number of articless, cited below and incorporated herein by reference:
Optical Thickness Measurement of SIO2-SI3N4 Films on Silicon, Reizman, et al., Soild State Electronics, Vol. 10, pp. 625-632 (1967); PA1 IOTA, a New Computer Controlled Film Thickness Measurement Tool, Konnerth, et al., Solid State Electronics, Vol. 15, pp. 371-380 (1972); PA1 Polycrystalline Silicon Film Thickness Measurement from Analysis of Visible Reflectance Spectra P. S. Hauge, Journal Optical Society of America, Vol. 69, No. 8, pp. 1143-1152, (August, 1979).
It has been known, for example, from the above IOTA article, to use a computer controlled system to provide rapid data acquisition and subsequent on-line reduction of the data in transparent thin film thickness measurements. This earlier system and subsequent systems have used randomized bifurcated fiber optic bundles aligned normal to the sample under inspection, the output being conveyed to a photodetector, A/D converter, interface and data reduction computer. However, such systems were lacking in both speed and accuracy.
Therefore, it is an objective of this invention to provide a modified optical system capable of rapidly scanning the illumination output of the bifurcated bundle across the surface of the wafer under test.
Further, a significant problem with the accuracy of the system resides in the accuracy of the analysis of the signal output of the photodetector. This receives the reflected illumination from the surface of the film being measured by way of the bifurcated fiber optic bundle and a scanning monochromator. As is well known, the scanning monochromator receives the light from the fiber optic bundle through an input slit, and reflects it off a first mirror onto a rapidly rotating grating. The reflection off the rotating grating during the period of a critical angle of rotation of the grate is passed to a second mirror and then to an exit port to a photodetector. It is essential to the accuracy and speed of analsis of the data that means be provided for accurately coordinating the rotational position of the rotating grating with triggering of the A/D converter which receives the output of the photodectector.
It is an objective of this invention to electronically provide this coordination, in order that the sample pulses applied to the clock input of the A/D converter and consistently aligned with the beginning of receipt of the output of the photodetector created by each single rotation of the scanning monochromator grating.
Because noise (e.g., 60.about.noise) can significantly degrade the accuracy of the system, it is a further objective of this invention to reduce or eliminate noise in the system.
Another problem with known systems is that the desired level of system accuracy is so high that variation in or degradation of system components can have a measurable negative effect. The most serious problems are from changes in output of the light source or the sensitivity of the photodetector.
An objective herein is to incorporate means for correcting for component drift or aging in the light source detector circuit.
The above objectives are achieved by providing a fiber optic bundle whose end face is substantially perpendicular to the wafer under inspection so that the emitted light travels in a path parallel to the surface of the wafer. A pentamirror system is provided to reflect the emitted light along a path which intersects the wafer at right angles. The return reflectance is reflected back through the same mirror system and onto the face of the fiber optic bundle. To prevent exact imaging of the return reflectance onto the emitting optics, the system is intentionally slightly defocused, so that the reflectance return is slightly blurred. Thus, each returning light image overlaps onto the neighboring image. The mirror reflecting system is mounted on a carriage, to move the analysis spot over the surface of the wafer. To convey the incident and reflected light to the sample, a pair of mirrors is provided. Both mirrors are carried on the carriage. The lenses are focused to collimate the light passing between the lenses.
The light return passes through the bifurcated bundle to a scanning monochromator of known design and to a photodetector whose output is read by an A/D converter. A novel circuit is provided for driving the motor which rotates the grating in the scanning monochromator while detecting the position of the grating relative to the mirrors. INDEX and PHASE signals are provided from an encoder coupled to the motor. The INDEX signal is provided first to the motor drive circuit, to be mixed in a phase-locked loop with the driving voltage signal to control the rotational position of the motor and enabling the correction for 60 cycle to prevent noise interference with the measurement. The same INDEX signal is coupled to a counter which generates the trigger signal to the A/D converter. The counter is provided with both the INDEX signal and the high frequency clock signal which will be used to sample the photodetector output. The clock signal itself is generated by a voltage controlled oscillator driven by a PHASE signal taken from the encoder. As a result of using the same INDEX signal to rotationally position the motor, and generate the trigger pulse for the A/D converter, the high frequencey sample pulses consistently occur in the same phase relationship with the output signal of the photodetector, so that the data reduction scheme is always sampling the same portions of the reflected visible spectra returned from the thin film under test.
The circuit for correcting for component drift and aging includes means for providing a sample of the same light used to illuminate the wafer under inspection to the same detector circuit used for the analysis. By integrating this sample and using it as a reference level for comparison of all data points derived from the spectral reflectance from a succession of images of the same wafer, the effect of componet aging is minimized or eliminated.
The use of this electronic system has enabled thin film measurements of very high accuracy.